1. Field of the Invention
This invention relates to an antenna device of a compact, thin design having wideband frequency characteristics.
2. Description of the Related Art
Recently, mobile satellite communications, using a mobile means (e.g. an airplane, a ship or a car) and a communication satellite, have come into wide use. With the spread of the mobile satellite communications, there has now been a stronger demand for a compact, high-performance antenna. As is well known, in order to achieve a compact antenna design, there has been proposed a meandering-type antenna in which a radiation element of the antenna comprises a wire-like conductor formed or bent into a meandering configuration. One example of such antenna is disclosed in JP-A-6-90108.
Generally, the frequency bandwidth of a conventional antenna is about several % in terms of the specific band, and when the length of a radiation element was reduced so as to achieve a compact design, a problem was occurred that the band was further narrowed. When the transmitting band and the receiving band were larger than the specific band thereof, a problem was occurred that a plurality of antennas for transmitting and receiving purposes were required.
FIGS. 13A to 13B are a plan view showing the construction of a conventional antenna, FIGS. 13B and 13C are views showing the installation of the conventional antenna, and FIG. 13D is a graph showing resonance characteristics of the conventional antenna. The resonance frequency of the conventional antenna is a single resonance, and therefore the conventional antenna can not deal with a plurality of frequency bands (that is, the frequency band between 137.0 MHz and 138.0 MHz for a down-line and the frequency band between 148.0 MHz and 150.05 MHz for an up-line) assigned to a mobile satellite communication system which effects a ground-satellite-ground data communication using a satellite orbiting in a low orbit. Namely, the resonance frequency fr is determined by the length L of a radiation element, and this has resulted in a problem that the resonance occurred only for the single frequency.
In the installation of the antenna on a mobile means such as a vehicle, it is desirable that the antenna should have a low posture (reduced antenna height) in order to reduce the wind pressure, acting on the antenna open surface, and also to prevent damage to the antenna upon contact with other objects. Particularly, when the antenna is installed on a container, the antenna height is about 0.5 m as a result of the stacking of the containers even if the above 1/4-wavelength grounded-type antenna is used, and therefore the installation of the antenna is impossible. If the conventional antenna, shown in FIG. 13A, is mounted vertically on the vehicle body as shown in FIG. 13B, in addition to the above bandwidth problem, further problems concerning the reduction of the wind pressure and the damage to the antenna upon contact with other object will be occurred. If the antenna is mounted horizontally on the vehicle body as shown in FIG. 13C, the impedance is lowered as the antenna approaches an electrically-conductive panel or plate of the vehicle body, and also the resonance frequency is shifted, so that the impedance matching between the antenna and the feeder line is adversely affected, which has resulted in a problem that the transmitting and receiving operation can not be operated.